1. Field of the Invention
The present invention relates to a reflecting plate to be used in a matrix display apparatus for reflecting an incident light, and a method for manufacturing the reflecting plate. More particularly, the invention relates to a reflection type liquid crystal display apparatus for effecting a display by reflecting an incident light on pixel electrode surfaces but without using any backlight, and a method for manufacturing the liquid crystal display apparatus.
2. Description of the Related Art
Of liquid crystal display apparatuss, the reflection type for effecting a display by reflecting an incident light coming from the outside has been noted for its low power consumption because of no necessity for a backlight as a light source and for its thin and light construction. In order to acquire a brighter display in the reflection type liquid crystal display apparatus, it is necessary to prepare a reflecting plate which has optimum reflecting characteristics for increasing the intensity of light to be scattered to the observer. For this necessity, the formation of a reflecting plate which has an uneven surface capable of realizing the "paper white" has been accepted as an important technique.
Japanese Unexamined Patent Publication JP-A 5-323371 (1993) has disclosed a technique for forming uneven spots on a surface of the reflecting plate with a photosensitive resin by the photo process. FIGS. 9A and 9B are top plan views showing a mask 1b of the prior art to be used for forming the uneven spots on the surface of the reflecting plate. FIG. 9A is a top plan view showing one pixel 61 of the mask 1b. FIG. 9B is a diagram showing an arrangement relation of unit patterns of the mask 1b.
The shape of the pixel 61 of the mask 1b is so designed that about two hundreds of circular regions 62, for example, are arranged at random so as to form the uneven spots for suppressing the interference of the reflected lights. The pixel 61 is a square having sides 63 of 100 .mu.m to 1,000 .mu.m, for example, and has a unit pattern, as designated by a symbol 64. The mask 1b is designed to determine the entire arrangement of uneven spots by repeating the mirror reflections of the unit pattern 64.
The method of manufacturing the reflecting plate having the uneven spots will be described by using the masks 1b. FIG. 4 is a perspective view for explaining an exposure step in the general reflecting plate manufacturing method. A photosensitive resin film 17 is formed over a predetermined substrate 10. A spherical exposure lamp 18 of a stepper device is arranged above the photosensitive resin film 17 of the substrate 10 to expose the photosensitive film 17 through a mask 1. The area to be irradiated by one exposure is dependent upon the size of the exposure lamp 18 and is no more at present than a size of a diagonal length of about 12.7 cm at the most. When a reflecting plate having a diagonal length of about 12.7 cm or more is to be manufacture, a predetermined number of exposure steps are repeated by moving the location of the mask 1 or the substrate 10. By one exposure, for example, only an exposure face A is exposed in the substrate 10. After this, different exposure faces B to H are sequentially exposed by moving the mask 1 or the substrate 10. By causing the circular regions 62, as owned by the conventional mask 1b used as the mask, to act as light shielding portions, for example, the regions of the photosensitive resin film 17 other than the circular regions 62 are exposed to the light.
When the exposed photosensitive resin film 17 is developed with a developer, circular columns are formed on regions corresponding to the circular regions 62. These circular columns are rounded to convexes having rounded surfaces by a heat treatment at 120 to 250.degree. C. All over the surface of the substrate 10 is formed a light reflecting film made of a metallic thin film, so as to cover the convexes. This light reflecting film is formed by the convexes into a continuously curved surface having gently conical uneven spots. The reflecting plate thus formed to have the uneven surface increases the intensity of light directed toward the observer so that it provides a bright paper white display when used in the reflection type liquid crystal display apparatus.
The mask 1b of the prior art is designed by repeating the mirror reflections of the unit pattern of the pixel 61 having the circular regions 62, as described above. The reflecting plate is made convex by the mask 1b so that the reflection type liquid crystal display apparatus utilizing the reflection plate can provide the bright paper white display. In case the reflection plate is large-sized, however, the exposure step is repeated plural times, and accordingly the reflecting characteristics are lowered with the result that the display quality is lowered in the display apparatus. That will be detailed in the following.
"FIG. 10 is a diagram illustrating an exposure intensity distribution by the exposure lamp 18 of the stepper device. By joining the points each having an equal exposure intensity in a region 23 to be exposed with curves, the portions of the high and low exposure intensities are expressed with contour lines 24 expressing gentle uneven spots. The exposure intensities become higher as arrows 25 come closer to a region 26 and the weaker as the arrows 25 come the closer to a region 27. This difference in the exposure intensity causes a dispersion of about 3% in the exposed region 23 and accordingly a nonuniformity in the exposure."
FIG. 11 is a diagram illustrating an optical intensity distribution of the case in which the exposure step is repeated plural times in different positions. For example, the exposures are executed in the order of exposure faces A, B, C and D. The exposure intensity gently changes for the exposure of only the face A but abruptly changes at each seam 28 between the faces for the exposures of two or more times in different positions. This abrupt change in the exposure intensity is observed as the exposure nonuniformity.
FIGS. 12A and 12B are perspective views illustrating a difference in shape between convexes 21 due to the exposure intensity. FIG. 12A shows the case in which the exposure intensity is high, and FIG. 12B shows the case in which the exposure intensity is low. Each circular column 20 to be formed on the surface of the substrate 10 is the thinner for the higher exposure intensity. By the heat treatment of the circular columns 20, the convexes 21 are rounded to have smooth surfaces. If the convexes 21 have a height d, they are the steeper for the higher exposure intensity and the gentler for the lower exposure intensity. The convexes 21 are given the different shapes as a result of the exposure nonuniformity.
FIGS. 13A and 13B are graphs illustrating the difference in the reflecting characteristics between the convexes 21 due to the exposure intensity. FIG. 13A illustrates the case in which the exposure intensity is high, and FIG. 13B illustrates the case in which the exposure intensity is low. The abscissa indicates an azimuth angle, and the ordinate indicates a reflection intensity. The reflection intensity is different at the portions of high and low exposure intensities because of the different shapes of the convexes 21. Specifically, the change in the reflection intensity is small over a wide range of visual angle at the portions of high exposure intensity and large over a range of visual angle at the portions of low exposure intensity.
FIGS. 14A and 14B are diagrams illustrating the change in the reflecting characteristics of a reflecting plate 71 in the case of using the conventional mask 1b in different positions. FIG. 14A illustrates a pattern of the reflecting characteristics, and FIG. 14B illustrates the reflecting characteristics of the exposure faces A and B. Reference numeral 65 indicates the state of the smallest change in the reflection intensity, which gradually becomes larger in the sequence of numerals 66, 67 and 68. The exposure faces A and B are formed of the plural pixels 61 which have the reflecting characteristics indicated by the numerals 65 to 68. Broken lines 69 are the contours indicating the exposure Intensity distribution by the exposure lamp 18. A region 69a is a region of much exposure, and this exposure becomes less in the sequence of regions 69b and 69c. By the first exposure, the face A is exposed, for example. By this single exposure, the uneven spots change continuously gently for the exposure nonuniformity, as indicated by the broken lines 69. According to this change, the reflecting characteristics change gently, too, so that the display nonuniformity, as might otherwise be caused the interference color, is not observed.
However, when the exposure step is repeated plural times on the faces A and B, for example, the nonuniformity of a seam 70 between the faces A and B is abruptly changed due to the abrupt change In exposure intensity. As a resulting the reflecting characteristics abruptly change in the seam 70 so that the difference in the reflection distribution is observed as an nonuniformity in the brightness. In the case of the display, the seam 70 is observed as a line so that the display quality is lowered.
FIG. 15 is a diagram illustrating the reflecting characteristics of the case in which the parallelism of an incident light is high in the conventional reflecting plate 71. By the repetitions of the identical unit patterns of the mask 1b, the individual pixels 61 of the reflecting plate 71 have the repetitions of the patterns which are identical in uneven spot shapes 72 and reflecting characteristics. For the parallel beam, therefore, the parallelism of the reflected lights also becomes high so that the interference color by the reflected lights is observed to seriously degrade the display quality.